What causes a face to be created only partially?
There are a lot of double vertices around the face, which meant that the selected edges around the missing triangle were not actually connected to the rest of the selection.
To fix this, select the vertices around that area, or select all (A) and press WR to remove doubles.
You seem to have a lot of doubles, it removed 305 vertices for me. Perhaps an accidentally applied edge split modifier or edge split operator (CtrlE&gt;Edge Split or D)? It's possible to split edges without.
Identifying doubles:
You can tell that the upper left edge is not connected to the selected vertex because the orange gradient that normally appears along selected edges is not visible.

After merging the double vertices with Remove doubles it looks like this:

Now fill will work as expected: